An image sensor is a semiconductor device to transform optical images into electrical signals. The image sensor may be classified into a charge coupled device (CCD) type or a complementary metal oxide semiconductor (CMOS) type. A CIS (CMOS image sensor) refers to a CMOS type image sensor. The CIS may include a plurality of two-dimensionally arranged pixels. Each of the pixels includes a photodiode (PD). The photodiode serves to transform an incident light into an electrical signal.